<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Null和Unidentified</title>
</head>
<body>
<script type="text/javascript">
  var a = null;
  console.log(a);//
  console.log(typeof  a);//object

  var b ;
  console.log(b);//
  console.log(typeof  b);//undefined
</script>
</body>
</html>